2 Answers
2

Generally all extensions are bad. If they affect how the browser behaves they are fingerprintable and therefor reduce your anonymity because your browser no longer acts like other peoples.

Firstly, most of these addons work by blacklisting which is always insufficient and flawed. Infact a study of such addons found there were cases that it always missed and that it always missed new, emerging fingerprinting techniques. This means that rather than improving your privacy, in the case of Tor Browser they are harming it. To get privacy on Tor Browser use 'New Identity', it will protect you better than any addon.

Secondly, Ghostery is proprietary software. There is no source code available, you will not be able to tell what it is doing without some reverse engineering effort.

So you should not be using Ghostery. If you want to block ads, use an adblocker but don't think that addons will stop tracking, they absolutely will not. At best they reduce it, in many cases they will make it worse.

"Thirdly, Ghostery sells you information to advertising companies, presumably the very thing you were intending to avoid" This is only true if out opt-in. It is very easy to opt-out. Do you do any research?
– JuliaJun 3 '17 at 22:15

It's a good thing to have, so the answer is YES: the neccessity of using Ghostery is not eliminated by using non-direct TCP transport(i.e. using Tor instead of direct clearnet) - it does not change the end point, the website you're visiting. And - it's not eliminating tracking scripts like Google Analytics. So Ghostery is must-have thing. Also consider using a Perspectives Project to verify HTTPS certificates, a good thing to have.

No these are not must have things and are bad for anonymity.
– cacahuatlNov 11 '16 at 21:06

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I don't think it hurts at all to have Ghostery installed in the Tor browser but something that I've noticed which is interesting is that Ghostery does not detect any trackers in the Tor browser for the same sites that it would find several trackers in Safari for example. It seems that Tor is doing a fair job of stopping the trackers in itself! But I will keep it installed and see how things go for a little longer:)
– user15015Nov 11 '16 at 21:11

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And thank you @AlexeyVesnin I will add Perspective Project to my Tor browser!
– user15015Nov 11 '16 at 21:13

@canonizingironize it really depends on your use-case. If you're trying to be truly anonymous, add-ons make your fingerprint unique. If you're just trying to get past a school firewall, or protect against snooping on an open Wi-Fi network, you could just use Tor as a proxy for your default browser.
– SuperSluetherNov 11 '16 at 23:09